1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a laser printer or a copier, or a transfer roller used in an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a latent image is formed on a photosensitive drum serving as an image bearing member. The latent image is developed using toner. The developed toner image is electrostatically transferred from the photosensitive drum to a transfer medium in a transfer nip portion formed by the photosensitive drum and a transfer roller.
In general, such a transfer roller used in an image forming apparatus is manufactured by forming a foaming sponge layer around a metal core bar serving as a shaft. The foaming sponge layer functions as a resistive layer or an elastic layer. Since a transfer roller of the foaming sponge type has a soft and elastic surface, the transfer roller can uniformly maintain a transfer nip portion N in the length direction (the axis direction of the transfer roller) when the transfer nip portion N is formed by pressing either end of the transfer roller in the length direction.
However, when a transfer roller of the foaming sponge type is manufactured, a foaming step is required during manufacture. Immediately after the transfer roller is manufactured, the accuracy of the outer shape is nonuniform. In order to limit the outer shape of the transfer roller of the foaming sponge type to a predetermined size, a step of making the accuracy of the outer shape uniform by polishing the surface of the transfer roller is required.
To solve such a problem, Japanese Patent Laid-Open No. 2008-298855 describes a method for employing a solid transfer roller as a transfer roller 5. The solid transfer roller has a surface layer made of a solid rubber formed from a non-foaming rubber, a resin, or a mixture of a non-foaming rubber and a resin. During a manufacturing stage, an error in the accuracy of the outer shape is less likely to occur for a solid transfer roller than for a transfer roller of the foaming sponge type. Thus, the outer shape of a solid transfer roller is easily formed to have a predetermined size.
However, when a solid transfer roller is employed, the following problem arises. Since a solid transfer roller has a hardness higher than that of a transfer roller of the foaming sponge type, it is difficult to ensure that the transfer nip portion N has a wide width in the rotation direction of the transfer roller along the length direction of the transfer roller. This is because the elastic layer of a solid transfer roller that has a hardness higher than that of a transfer roller of the foaming sponge type does not easily collapse even when the same pressure force is applied. Accordingly, as compared with a transfer roller of the foaming sponge type, the entire width of the transfer nip portion N of a solid transfer roller tends to be small along the length direction. In particular, the middle area of the transfer nip portion N in the length direction may be too narrow with respect to the end areas of the transfer nip portion N. In such a case, a desired electrical current does not flow in the middle area of the transfer nip portion N and, therefore, the color density of an image in the middle area tends to be lower than that in the end areas.